This invention relates to a surface acoustic wave filter (hereinafter SAW filter) for filtering electrical signals and especially to a high-performance, wide band SAW filter.
Various kinds of SAW filter have been widely used in different kinds of electronic equipment. With conventional SAW filter manufacturing techniques, however, it has been difficult to manufacture a high-performance, wide band SAW filter such as a vestigial side band (VSB) filter for a television transmitter. The VSB filter, for example, is required to have a flat frequency amplitude (transfer) characteristic (see curve A in FIG. 1) over a large relative band width, a flat group delay time characteristic (see curve B), and a shape factor close to 1 as shown in FIG. 1. Shape factor is defined as the ratio of the -30 dB bandwidth to the -3 dB bandwidth, 0 dB being the attenuation at the central frequency f.sub.o. The closer to 1 the shape factor becomes, the more sharply the characteristic rises and falls. Group delay T is defined by the following equation EQU T=dp/df (1),
where p is the phase difference (in Hz) between a given wave and waves at the central frequency f.sub.o, and f is frequency in Hz. Ideally, T should be constant (flat) over the filter's pass band. Relative band width is the ratio of the -3 dB bandwidth to the central frequency f.sub.o.
The inventors of the present invention attempted to design such a VSB filter having as an input transducer in an interdigital comb-like transducer in which the lengths of the overlapped portions of electrode fingers are varied in accordance with their location along the direction of surface wave propagation. The conventional design technique is as follows:
(1) Establish a desired transfer characteristic, for example as shown in FIG. 1.
(2) Determine the inverse Fourier transform of this transfer characteristic, i.e., the desired impulse response of the filter, for example as shown in FIG. 2.
(3) Arrange the electrodes in the form of this desired impulse response, as shown in FIG. 3.
FIG. 3 shows an example of a SAW filter designed according to the conventional apodizing method. In FIG. 3 is shown a filter which has a normal transducer 12 and an apodized transducer 14, both of which are mounted on a piezoelectric substrate 16. An apodized transducer with a plurality of electrode fingers arrayed at an equal pitch (spacing) is used as the input transducer, while a normal transducer, also with a plurality of equally-spaced electrode fingers, is used as the output transducer.
When the impulse response of this conventionally-designed filter was measured, it was found to be asymmetrical from right to left, as shown in FIG. 4, where the horizontal and vertical axes represent time and voltage respectively. The distinctive feature of the result of this measurement was that the maximum amplitudes of the right-hand side lobes 24 were smaller than those of the corresponding left-hand side lobes 22. If one compares, for example, the maximum amplitudes A1 and A2 of corresponding side lobes 22.sub.2 and 24.sub.2, which are the second lobes from main lobe 26, he notes that A1 is larger than A2. For any such corresponding pair of side lobes, the left-hand side lobes were the larger. These results mean that the filter designed according to the conventional design procedure did not match the required transfer characteristic shown in FIG. 1, for it is known to one skilled in the art that if the impulse response waveform of the filter is asymmetrical, a flat group delay time characteristic cannot be obtained, and if the pitches of the points where the waveform of the impulse response intersects the horizontal axis are not uniform, the transfer characteristic will not be flat. (The pitches of the intersections correspond to the pitches of electrode fingers of the transducer.)